Synthetic Resin Mortars

A combination of synthetic resin binders and fillers (aggregate) similar to cement creates, after curing, a mortar-like consistency building material with properties unattainable with traditional materials. This mortar, also called synthetic resin concrete, has striking properties: extremely high mechanical strength, chemical resistance, short curing time, and excellent adhesion to traditional building materials. For the properties and economics of synthetic resin concrete, workability with binder-sand ratio is important. The lower workability limit is given with a 2% resin content, but such a material will likely fall short of greater interest because it lacks sufficient strength and is permeable to air and liquids. As resin ratio increases, strength increases. This strength is achieved most at 15% binder; therefore, more binder is unnecessary because more resin reduces strength. Nevertheless, synthetic concrete with 20% binder reaches approximately 75.0 N/cm² compressive strength and approximately 25.0 N/cm² shear strength, while normal concrete only achieves 25.0 N/cm² compressive strength and 7.0 N/cm² shear strength. Synthetic resin mortars with higher resin content may be of special practical interest because they have greater fluidity and self-leveling properties. This provides good workability, enabling high floor coverage rates. The most common synthetic resin mortar consists of 10–15% resin and 85–90% sand by weight. When surfaces and structures are continuously exposed to severe conditions, paints and varnishes prove insufficient for lasting protection. Here synthetic resin mortars can be recommended because, when appropriate resins are used, very good mechanical strength and good chemical resistance can be achieved. There are two main synthetic resin mortar groups: 1. Mortars applied with a trowel, 2. Self-leveling mortars.

The two groups are characterized by differences in the amount of binder content and the particle size composition of fillers.

Trowel-applicable mortars generally have 85% or higher filler content and are normally used at 5–10 mm thicknesses. Synthetic resin mortars prepared and cured according to these formulations have the following properties: • High mechanical strength, such as compressive strength, • Transverse and longitudinal impact resistance. Low thermal expansion coefficient means the mortar can be laid easily at low temperatures. These can only be applied with an appropriate tool like a trowel. Some formulations must be compacted well after application. Specially prepared formulations can enable synthetic resin mortars to be applied to vertical surfaces. Self-leveling mortars are mortars containing 85% or less filler and are normally used at 3–6 mm thickness. Synthetic resin mortars produced according to these formulations can be spread rapidly; on horizontal surfaces only rough leveling is required. These mortars are less impact-resistant than trowel-applied mortars. Unlike mortars with low binder content, their surfaces are smooth, glossy, and impermeable to liquids. If any surface tackiness remains after hardening, it can be cleaned with water. To create a non-slip surface, sand should be sprinkled on before the coating cures. If the self-leveling mortar contains a high proportion of resin, after hardening it has a thermal expansion coefficient that can be three to five times greater than that of concrete or steel. This means the mortar cannot be used on surfaces found at low temperatures (0°C or below). When the coating is thicker than 2 mm, stress conditions may arise that can cause the coating to crack. However, these stresses are rarely observed in coatings 2 mm thick. Producing a synthetic resin mortar is quite straightforward. If the mixture ratios of resin and hardener components are well prepared and the mortar is properly mixed in an anhydrous environment, satisfactory results will be achieved.

Standard values for application are given below; they may vary slightly depending on surface conditions.

Coating quality depends on proper working conditions. Resin systems have optimal properties at the mixture ratios given in formulations. Only a completely homogeneous mixture of the two components reacts as desired throughout the mortar process. Therefore, one must not forget the necessity of complete and thorough mixing. To make a good synthetic resin mortar, mechanical mixers with edge scrapers and typically 50–80 litre capacity are required. Gravity mixers do not have sufficient mixing effect. First, appropriate amounts of epoxy resin and hardener are uniformly mixed in a mechanical mixer. Then the filler is mixed and kneaded until thoroughly wetted. Mixing takes approximately five minutes. To prevent excessive air entry during mixing, 0.5% silicone oil can be added over the binder. Some of the most suitable silicone oils may be those from companies such as Wacker, Bayer, or Byk. The addition of silicone oils is particularly recommended for self-leveling synthetic resin mortars because entrained air passes to the surface, creating bubbles after the self-leveling coating spreads.

Under normal ambient conditions, the binder has a limited pot life ranging from 20 minutes to six hours depending on the type of hardener. Pot life is also significantly affected by the batch size and ambient temperature.

Fairly large batches are divided into smaller batches after mixing to accelerate the reaction and prevent overheating. Good adhesion of the binder can be a disadvantage during spreading of synthetic resin mortar, depending on the formulation. After a time, the mortar sticks to the trowel, making smooth finishing difficult. It is helpful to apply a brush-moistened solvent to the finishing machine; for this purpose, 2-ethoxyethanol is effective. This difficulty does not arise when paraffin oil is present in the formulation. Aeration of the mortar with silicone oil also reduces adhesion to the trowel. When mortars with low resin content, such as 10%, are spread, a primer or bonding agent is recommended. The resin/hardener mixture used in the mortar but without filler can be used as a bonding agent, applied thinly to the surface before laying the mortar. In practice, it serves as an adhesive between the substrate and the low-binder mortar. A filled bonding agent has application advantages. Quartz dust and Aerosil are suitable fillers that prevent a spotted, oily appearance of the adhesive binder. The adhesive binder can be coated with synthetic resin mortar immediately after application. However, if the mortar contains approximately 15% binder, an adhesive binder is not necessary.

Great care must be taken in preparing the surface before applying synthetic resin mortar or adhesive binder. Only a perfectly prepared surface guarantees perfect coating adhesion.

To enable weak concrete substrate to better resist mechanical stresses, the synthetic resin concrete must be thick enough to support the weak concrete, meaning at least 15–20 mm thickness. Conversely, on a good, solid substrate, 5 mm thickness is normally entirely sufficient. The viscosity of resins and hardeners and pot life depend on the temperature present in the environment. To ensure thorough mixing, resins and aggregates should not be at temperatures lower than 15°C. This temperature is also the lower limit of the curing temperature for non-accelerated binder systems. When low curing temperatures are unavoidable, they must be accelerated with accelerated hardeners. At normal temperature, synthetic resin concrete fully cures within 24–28 hours depending on the reactivity of the binder. Under these conditions, the concrete is strong enough to bear minor mechanical loads after just 12 hours. Full mechanical loading is possible after 3–7 days. Concrete should never be exposed to chemical influences for less than seven days. Conditions differ with an amino polyimidazoline-type hardener of low reactivity and long pot life with an amine value of 350–400 and AHEW of 93, because it only gains measurable strength after more than 24 hours of curing. Light mechanical stress can be applied after two or three days. However, full mechanical and chemical service conditions can be applied after seven days because a system containing this type of hardener undergoes delayed initial cure during the remaining period.   M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya Ltd. Şti.      

References 1 Schering Industrie-Chemikalien. 2. H.Lee, K.Neville handbook of Epoxy Resins. 3. Huntsman Advanced Materials (Switzerland) GmbH Klybeckstrasse 200 P.O. Box 4002 Basel Switzerland 4. Epoxy Polymers, Edited by Jean-Pierre Pascault and Roberto J. J. Williams, WILEY-VLH Verlag umbH. 5. Paint and Coating Testing Manual Fifteenth Edition of the Gardner-Sward Handbook, Joseph V. Koleske.